1. Field of the Invention.
This invention relates generally to an apparatus and method for detecting and measuring Hg+ and Hg2+ in solution phase samples. More particularly, this invention relates to use of a selective electrical probe for Hg+ and Hg2+ containing a chelating or clathrating agent for mercury covalently bonded within an organic polymer matrix.
2. Related Art.
There is considerable interest in rugged, portable, hand-held, battery-powered sensors that yield results comparable to laboratory-based instruments. Mercury is of particular interest due to its toxicity, volatility, and the ease with which it is converted to forms that enter the environment and, ultimately, the food chain. Mercury in soil and water is taken up by plants and by animals that feed on the plants. In order to rapidly assess mercury contamination in the field, a rugged analytical device capable of detecting mercury at the ppb to sub-ppm level has long been sought.
Several potentiometric mercury electrodes have been demonstrated having detection limits as low as 2 ppm. Because environmental mercury contamination is usually in the ppb range, these potentiometric probes have not proven useful for field analysis. Most of these probes rely on random casts of a chelating agent to achieve mechanical entrapment of the chelating binding site in a liquid membrane. Loss of the chelating agent from the membrane by leaching over time results in fairly short working lifetimes, on the order of a few weeks.
An alternative approach for mercury measurement is based on substitution of Hg2+ for Ag+ in a halide crystal deposited on a graphite substrate. Detection limits for these substitution-type electrodes have been reported in the 2 ppb range, after a preconcentration step, which still makes them less useful for analysis in the field. Furthermore, this type of probe experiences significant interference from other ions such as cadmium (II), lead (II) and silver (I), which ions can also substitute to some extent into the crystal lattice, giving a false positive signal for mercury. These interferences reduce the selectivity and, ultimately, the utility of this measurement device.
Much work has been done in the general area of chelation of metals for electrochemical analysis. One of the more successful examples of this type of analytical device is the potassium ion selective electrode (ISE). This probe is based on incorporation of an 18-crown-6 ether macrocycle (e.g., Valinomycin(trademark)) into a polymeric membrane. The crown ether exhibits selective coordination for potassium even in the presence of sodium. The calcium ISE relies on a coordination interaction between calcium ions in the test solution and organophosphates suspended in a liquid ion exchange membrane coated onto a potentiometric probe. As the calcium is bound or released from the membrane, the surface potential changes in a fashion related to the natural logarithm of the calcium concentration.
Still, there is a need for a simple, reproducible and inexpensive probe for Hg+ and Hg2+ . Also, there is a need for such a probe with a detection limit less than or equal to about 2 ppb with a long working lifetime, on the order of more than one year. Also, there is a need for such a probe with an amperometric response which is linear with mercury concentration over several orders of magnitude, and which is not significantly affected by dissolved oxygen, or by cadmium, lead or silver. The present invention addresses and satisfies these needs.
The present invention is an apparatus and method for detecting and measuring Hg+ and Hg2+ in samples. The invention comprises a copolymer of semi-conductive organic material including derivatized and underivatized monomers. The derivatizing functional group is a chelating and/or clathrating agent, chosen based on the agent""s selectivity for coordination with the analyte metal, mercury. Preferably, the invented copolymer is deposited on a conductive substrate in order to construct a long-lived, economic instrument probe with a detection limit of less than about 2 ppb mercury in a fluid sample.
The preferred derivatizing functional group is 1,4,10-trioxa-7,13-diazacyclopentadecane, commercially available as Kryptofix-21(trademark). This crown either has a high formation constant for complexations with mercury and is capable of stabilizing both members of the Hg+/Hg2+ couple.
To prepare the preferred apparatus of the invention, thiophene, or other, similar monomer, is derivatized by covalent attachment thereto of Kryptofix-21(trademark) (1,4,10-trioxa-7,13-diazacyclopentadecane) in a position that will not be chain terminating during polymerization. The thiophene monomer and Kryptofix-21(trademark) are co-polymerized and electrodeposited by known techniques onto a conductive substrate, such as platinum or glassy carbon, to obtain a multi-layer polymer coating of desired thickness. The resulting coated electrode has selective coordination sites for mercury having formation constants of the order Kf=1016, compared to approximately Kf=103 for cadmium, Kf=105 for lead and Kf=105 for silver. Therefore, the resulting probe has the benefit of being very selective for mercury, compared to cadmium, lead and silver, and having a long working life due to the covalently bonding of the chelating agent in the probe surface.